Hitherto, various combustion catalysts for purifying the exhaust gas of automobiles have been developed. For example, combustion catalysts comprising a carrier made of alumina or the like having a catalytically active precious metal such as platinum, rhodium or palladium supported thereon are known.
Specifically, in Japanese Patent Application Laid-open No. 7-256113 there is disclosed a combustion catalyst for purifying exhaust gas, characterized in that a first coating layer having as a principal component thereof activated alumina containing a precious metal selected from the group consisting of platinum, palladium and rhodium is provided on a catalyst carrier, a second coating layer comprising an inorganic material having as a principal component thereof activated alumina not containing a precious metal component is provided on the first coating layer, and a third coating layer comprising an inorganic material having as a principal component thereof a zeolite powder that has been subjected to ion exchange with a metal is provided on the second coating layer to form a catalyst, and this catalyst is disposed on the upstream side of a catalytic converter, and a three-way catalyst is disposed on the downstream side of the catalytic converter.
Of existing combustion catalysts, it is known that in particular combustion catalysts having a precious metal as an active component as described above have high exhaust gas purifying activity. However, such combustion catalysts have the problem that the cost is high due to the precious metal component being expensive. In particular, in the case of using alumina or the like as a carrier, the cost becomes yet higher.
There are thus calls for the development of combustion catalysts that have high exhaust gas purifying activity but are less expensive than conventional ones.